Abstract: A reducing dross method of lead-free solder includes the steps of: producing master alloy of reducing dross which comprises Sn and 0.1 to 0.8 wt % P; analyzing the percentage of P of the lead-free solder to be modified, in order to reach 0.008 to 0.015 wt % P in the lead-free solder, adding the master alloy into the lead-free solder with the percentage of P less than 0.008 wt % or no P; then sampling the lead-free solder at regular intervals to determine the percentage and the percentage loss of P, if the percentage of P being less than a given value from 0.008 to 0.015 wt %, adding the master alloy to keep the percentage of P as 0.008 to 0.015 wt %.